Photobioreactors are a special field in the area of bioengineering, since the reactors therefor are not reactors being only adapted also for the area of chemical process engineering, wherein the optimization of the reaction procedures only comprises the transportation processes and mixing techniques. Rather, for photoreactors, the input of light and the distribution thereof within the reaction medium plays an important role, the more so as light is not dispersible in aqueous media, at least not now. Up to today, this technical challenge led to various designs for photobioreactors.
In principle, stirred-tank reactors (often with internal illumination), tubular reactors, (vertical) column reactors, and flat-plate reactors can be distinguished. Generally, the supply of light energy to the reaction medium occurs from outside, i.e. through a reactor wall being transparent for light. Reactors with such external supply of light can only difficultly be extended from a laboratory scale to an industrial scale, since the efficiency of the supply of light and thus of the photobioreactions depends on the surface of the transparent reactor wall and will thus not increase linearly with an enlargement of the reactor volume. An enlargement of the reactor volume will thus lead to decreasing reaction rates.
Another problem of such externally lighted photobioreactors is that the light conditions within the reaction medium, which contains turbid matter, together with the cells, are inhomogeneous, and this inhomogeneity will even increase with increasing cell density, i.e. with the progress of the reaction. Therefore, the mixing behavior in such photobioreactors also plays a very important role. In principle, it would be desirable to arrange for substantially identical reaction conditions within different volume units of the reaction medium.
Finally, the supplied light power cannot be increased infinitely, since in many phototrophous microorganisms a photoinhibition will occur as from a certain light intensity. This is consequently the maximum value in an arbitrary volume element of the reactor space, and in other volume elements the light power is then clearly lower, due to absorption, with the consequence of a lower photobiosynthesis performance.
A design known from the practice is a stirred-tank reactor, into the reaction chamber of which optical fibers are guided, which are supplied with light from outside. The light is guided through the optical fibers into the reaction medium and is emitted there. With a suitable arrangement of the ends of the optical fibers in the reaction chamber, a nearly homogeneous illumination of the reaction medium can be achieved. However, the optical fibers will disturb the mass transportation processes, convection and diffusion, for geometric reasons to a substantial degree, thereby then in turn the homogeneity of the distribution of the reactants within the reaction chamber being adversely affected, and thus ultimately the achievable reaction rates. An example of such a type of reactor is described in the document DE 10 2007 055 569 A1.
Another class of photobioreactors are the flat-plate reactors. Just as an example, reference is made to the document DE 10 2009 015 925 A1. A characteristic parameter of this design is the thickness of the at least one-side transparent reactors, which typically is in the range of 70 mm. They are operated analogously to so-called “airlift” reactors with an exposure to gas along the reactor bottom. This permits a mixing behavior, which with an optimum height and length of the reactor will come close to 80% of the mixing efficiency of an ideal stirred tank. Nevertheless, the mass transfer of CO2 and O2 is not very efficient due to the short lifetime of gas bubbles. This can be improved by different parameters, whereby a scale-up up to 133 1 becomes possible (see document EP 1 326 959 B1).
In principle, tubular reactors in biotechnology have the same disadvantages as in chemical process engineering. The biggest disadvantage with horizontal tubular reactors is that the reduction of substrate along the flow direction of the reaction medium, thus consequently the reaction rates being reduced, can be neglected for short tubes, since then the reaction or growth kinetics is in a first approximation of zero order. However, this describes only the main reaction of the process, for instance the chemical properties of CO2 in water can substantially affect the pH value, thus the growth being inhibited or slowed down. Further, the necessary removal of O2 from the reaction medium plays a role influencing the growth. All these problems are strongly increasing with increasing tube length. Nevertheless, such photobioreactors of up 500 km length and a photo-active volume of 600 m3 are known in the art. The reaction efficiency is however comparatively poor for the mentioned reasons and for an up-scale, the length increase and thus the technical efforts need to be increased in a disproportionate manner.
Another type of tubular reactor is formed by a vertical arrangement of the tubes, with the tubes being U-shaped and having in the range of the U-curve a gassing device. Thereby, further the mass transfer through the U-legs is caused. For an example of such a type of reactor in biotechnology, reference is made to the document DE 197 47 994 C1. With such type of reactor, a scale-up up to 120 1 is possible, but there is still a necessity of some improvement.
With regard to mixing behavior and mass transfer, a stirred-tank reactor would be ideal. Such stirred-tank reactors with transparent walls and external lighting are up to now used as photobioreactors on a laboratory scale only and not on an industrial scale, due to the problems of the homogeneous introduction of the light into all volume elements of the reaction chamber. The attenuation of the light follows an exponential law with the distance from the transparent reactor wall, the more so as also the extinction coefficient increases with increasing cell density. For a scale-up, therefore, it is basically always tried to increase the ration of (transparent) surface to volume, or to optimize it. In a not-turbid liquid, the light intensity of a cylindrical body lighted from all around would lead to an increased intensity in the center axis, similar to the effect of a condenser lens. In a cell suspension, then a cell density can be established, in which there is approximately everywhere in the cross section the same light intensity. This cell density is however not optimal in view of the achievable production performance. In principle, the above problems do not only exist in the field of the photobioreactors, but also in the field of the chemical reactors for photoreactions of chemical reactants.